Pyrazolyl-bridged iridium dimers.: 17.: Tetrakis(alkene)diiridium(I) complexes:: [Ir(η2-C2H4)2(μ-Cl)]2 as a precursor to [Ir(η2-C2H4)2(μ-pz)]2.: Stereochemically nonrigid behavior of the analogue [Ir(η2-C2H4)(η2-C2F4)(μ-Cl)]2

The known bis(ethene) dimer [Ir(eta(2)-C2H4)(2)Cl](2), 1, reacts with pyrazole in the presence of NEt3 to form the deep purple pyrazolyl-bridged diiridium(I) analogue [Ir(eta(2)-C2H4)(2)(mu-pz)](2) (2), which is however stable only in an atmosphere of ethylene gas. Reaction of 2 with MeI affords a pale orange product formulated as the d(2)(7) diiridium(II) adduct [Ir(Me)(eta(2)-C2H4)(2)(mu-pz)(2)Ir(I)(eta(2)-C2H4)(2)] (3) on the basis of its H-1 NMR spectrum, which provides no evidence for dissociation of 3 in solution (contrasting with the behavior of the corresponding adduct formed by the bis(cycloocta-1,5-diene) analogue of 2); like 2, 3 is unstable in the absence of excess ethylene. No congener of 2 could be obtained by similar treatment of the perfluoroethylene analogue [Ir(eta(2)-C2H4)(eta(2)-C2F4)Cl](2) (4) of 1, although 4 was found to be less unstable than previously suggested and has been fully characterized in solution below -70 degrees C by F-19 as well as 1H NMR measurements. Thus, in acetone (but not in toluene or methylene chloride), 4 exists in equilibrium with two species, 4A and 4B, formulated as isomeric, mononuclear, 16e solvento complexes [Ir(eta(2)-C2H4)(eta(2)-C2F4)Cl(S)], S = acetone; 4:4A:4B (-90 degrees C) ca. 67: 13:20. The slow-limiting F-19 NMR spectrum of 4, which has been assigned with the aid of F-19-F-19 COSY data,;unambiguously identifies a folded binuclear geometry that undergoes facile ring inversion of the chloro-bridged core, with Delta G(200)(double dagger) ca. 39 kJ mol(-1) estimated from the temperature-dependent NMR behavior. Coalescence data for signals attributed to mononuclear 4A are consistent with "propeller rotation" of the C2F4 ligand at d(8) Ir(I), Delta G(200)(double dagger) ca. 41 kJ mol(-1). Mononuclear, 18e, substituted cyclopentadienyl complexes Ir(eta(5)-C5H4R)(eta(2)-C2H4)(eta(2)-L) (5,R = COCO2Et, L = C2H4; 6, R = COOMe, L = C2F4) can also be synthesized by bridge-cleavage reactions of 1 and 4, respectively.